Engine retarders of the compression release-type are well-known in the art. Engine retarders are designed to convert, at least temporarily, an internal combustion engine of compression-ignition type into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted compression release-type engine retarder can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine in positive power.
Safety, reliability and environmental demands have pushed the technology of compression release engine retarding significantly over the past 30 years. Compression release retarding systems are typically adapted to a particular engine in order to maximize the retarding horsepower that could be developed, consistent with the mechanical limitations of the engine system. In addition, over the decades during which these improvements were made, compression release-type engine retarders garnered substantial commercial success. Engine manufacturers have become more willing to embrace compression release retarding technology. Compression release-type retarders have continued to enjoy substantial and continuing commercial success in the marketplace. Accordingly, engine manufacturers have been more willing to make engine design modifications, in order to accommodate the compression release-type engine retarder, as well as to improve its performance and efficiency.
In addition to these pressures, environmental restrictions have forced engine manufacturers to explore a variety of new ways to improve the efficiency of their engines. These changes have forced a number of engine modifications. Engines have become smaller and more fuel efficient. Yet, the demands on retarder performance have often increased, requiring the compression release-type engine retarder to generate greater amounts of retarding horsepower under more limiting conditions.
As the market for compression release-type engine retarders has developed and matured, the aforementioned factors have pushed the direction of technological development toward a number of goals: securing higher retarding horsepower from the compression release retarder; working with, in some cases, lower masses of air deliverable to the cylinders through the intake system; and the inter-relation of various collateral or ancillary equipment, such as: silencers; turbochargers; and exhaust brakes. In addition, the market for compression release engine retarders has moved from the after-market, to original equipment manufacturers. Engine manufacturers have shown an increased willingness to make design modifications to their engines that would increase the performance and reliability and broaden the operating parameters of the compression release-type engine retarder.
Functionally, compression release-type retarders supplement the braking capacity of the primary vehicle wheel braking system. In so doing, it extends substantially the life of the primary (or wheel) braking system of the vehicle. The basic design for a compression release engine retarding system of the type involved with this invention is disclosed in Cummins, U.S. Pat. No. 3,220,392, issued November 1965.
The compression release-type engine retarder disclosed in the Cummins '392 patent employs a hydraulic system or linkage. The hydraulic linkage of a typical compression release-type engine retarder may be linked to the valve train of the engine. When the engine is under positive power, the hydraulic linkage may be disabled from providing valve actuation. When compression release-type retarding is desired, the hydraulic linkage is enabled such that valve actuation is provided by the hydraulic linkage responsive to an input from the valve train.
Among the hydraulic linkages that have been employed to control valve actuation (both in braking and positive power), are so-called "lost-motion" systems. Lost-motion, per se, is not new. It has been known that lost-motion systems are useful for variable valve control for internal combustion engines for decades. In general, lost-motion systems work by modifying the hydraulic or mechanical circuit connecting the actuator (typically the cam shaft) and the valve stem to change the length of that circuit and lose a portion or all of the cam actuated motion that would otherwise be delivered to the valve stem to actuate a valve opening event. In this way lost-motion systems may be used to vary valve event timing, duration, and the valve lift.
Compression release-type engine retarders may employ a lost motion system in which a master piston engages the valve train (e.g. a push tube, cam, or rocker arm) of the engine. When the retarder is engaged, the valve train actuates the master piston, which is hydraulically connected to a slave piston. The motion of the master piston controls the motion of the slave piston, which in turn may open the exhaust valve of the internal combustion engine at a point near the end of a piston's compression stroke. In doing so, the work that is done in compressing the intake air cannot be recovered during the subsequent expansion (or power) stroke of the engine. Instead, it is dissipated through the exhaust and radiator systems of the engine. By dissipating energy developed from the work done in compressing the cylinder gases, the compression release-type retarder dissipates the kinetic energy of the vehicle, which may be used to slow the vehicle down.
Regardless of the specific actuation means chosen, inherent limits were imposed on operation of the compression release-type retarder based on engine parameters. One such engine parameter is the physical relationship of an engine cylinder valve used for compression release braking and the piston in the same cylinder. If the extension of the valve into the cylinder was unconstrained during compression release braking, the valve could extend so far down into the cylinder that it impacts with the piston in the cylinder.
There may be a significant risk of valve-to-piston contact when a unitary cam lobe is used to impart the valve motion for both the compression release valve event and the main exhaust valve event. Use of a unitary cam lobe for both events means that the relatively large main exhaust lobe motion will be imparted to the hydraulic linkage, or more particularly to the slave piston. Because there is typically little or no lash between the slave piston and the exhaust valve, input of the main exhaust event motion to the slave piston may produce a greater than desired main exhaust event.
Accordingly, there is a need for a system and method for avoiding the occurrence of valve-to-piston contact when a unitary cam lobe is used to impart the valve motion for both a compression release event and a main exhaust valve event. More particularly, there is a need for a system and method of limiting the stroke or displacement of a slave piston when a lost motion system is imparted with the motion from a main exhaust cam lobe.
One way of avoiding valve-to-piston contact as a result of using a unitary cam lobe for both compression release valve events and main exhaust valve events is to limit the motion of the slave piston which is responsible for pushing the valve into the cylinder during compression release braking. A device that may be used to limit slave piston motion is disclosed in Cavanagh, U.S. Pat. No. 4,399,787 (Aug. 23, 1983) for an Engine Retarder Hydraulic Reset Mechanism, which is incorporated herein by reference. Another device that may be used to limit slave piston motion is disclosed in Hu, U.S. Pat. No. 5,201,290 (Apr. 13, 1993) for a Compression Relief Engine Retarder Clip Valve, which is also incorporated herein by reference. Both of these (reset valves and clip valves) may comprise means for blocking a passage in a slave piston during the downward movement of the slave piston (such as the passage 344 of the slave piston 340 of FIG. 6). After the slave piston reaches a threshold downward displacement, the reset valve or clip valve may unblock the passage through the slave piston and allow the oil displacing the slave piston to drain there through, causing the slave piston to return to its upper position under the influence of a return spring.
A reset valve, such as the one disclosed in Cavanagh, may be provided as part of a lash adjuster or a slave piston. A reset valve may comprise a hydraulically actuated means for unblocking a passage through the slave piston to limit the displacement of the slave piston. In Cavanagh, compression release retarding is carried out by opening one of two valves connected by a crosshead member or bridge. A purpose of the reset valve used in Cavanagh is to reseat the exhaust valve used for the compression release event before a subsequent main exhaust valve event so that the rocker arm will not push down on an unbalanced crosshead during the main exhaust event and transmit a bending force to the crosshead guide pin or to the non-braking valve stem.
A clip valve, such as the one disclosed in Hu, may comprise a mechanically actuated means for unblocking the passage through the slave piston to limit the displacement of the slave piston. A purpose of the Hu clip valve is to enable a sharp hydraulic pulse to be applied to the slave piston to rapidly open an exhaust valve while maintaining an accurate limit on the extension of the slave piston.
FIG. 1 illustrates a system in which a cam section 110 is connected to valves 200 by both a hydraulic linkage 300 and a mechanical linkage 400. With reference to FIG. 1, the actuation provided by the hydraulic linkage 300, which may include a slave piston, during the main exhaust valve event may be further limited by providing the mechanical linkage 400 with a greater actuation ratio than that of the hydraulic linkage. For example, for each unit of linear motion input to the hydraulic and mechanical linkages, the hydraulic linkage may transfer 1.3 units of linear motion to the valve 200 while the mechanical linkage may transfer 1.5 units of linear motion. By differing the actuation ratios of the hydraulic and mechanical linkages, the mechanical linkage 400 may be able to make up the lash distance 410 and thereby dominate the actuation of the valve 200 during the main exhaust portion 114 of the cam lobe.
Use of a unitary cam lobe for both the compression release event and the main exhaust event may also result in excessive overlap between the opening of the exhaust valve for the main exhaust valve event and the opening of the intake valve for the main intake event. With reference to FIG. 3, when the main exhaust event is input to the slave piston, the exhaust valve motion may be represented by curve 520-620 and the overlap of the main exhaust event with the main intake event may be illustrated by the combined shaded areas 650 and 652. The overlap represented by areas 650 and 652 may dramatically reduce brake effectiveness because intake charge (mass) used for the subsequent compression release event may pass right through the cylinder and out the exhaust port.
Accordingly, there is a need for a system and method for limiting and controlling the overlap between the main exhaust event and the main intake event when a unitary cam lobe is used to provide both a compression release event and the main exhaust event.
There also remains a significant need for a system and method for controlling the actuation of the exhaust valve in order to increase the effectiveness of and optimize the compression release retarding event. Further, there remains a significant need for a system that is able to perform that function over a wide range of engine operating parameters and conditions. In particular, there remains a need to "tune" the compression release-type retarder system in order to optimize its performance Whereas, exhaust valve actuation for retarding that can be provided by the existing cam profiles (valve or injector) may not produce this result.